Introduction: Surgical overcorrections in unilateral fourth nerve palsies are generally less well tolerated than undercorrections. We investigated the angle of deviation in various gaze positions as a risk factor for overcorrection of moderate angle unilateral fourth nerve palsies undergoing two-muscle surgery.

Methods: The medical records of 45 patients, age 12 to 77 years, who underwent two-muscle surgery for unilateral fourth nerve palsy with 14-25 prism diopters (pd) of hypertropia in primary position were retrospectively reviewed. Patients with overcorrection, defined as any reversal of hypertropia at both distance and near, were compared to non-overcorrected patients for preoperative deviations in primary, side and near gaze as well as torsion.

Results: Of the 45 patients, 8(17.8%) experienced surgical overcorrection by 6 weeks. For overcorrected and non-overcorrected patients, the median deviation in primary position was 20.0pd and 18.0pd (p=0.39), 25.0pd and 25.0pd in contralateral gaze (p=0.53), 8.5pd and 16.0pd in ipsilateral gaze (p=0.029), and 17.0pd and 18.0pd at near (p=0.49), respectively. There was no difference in preoperative torsion between overcorrected and non-overcorrected patients (p=0.7666). Ipsilateral gaze of <10pd was associated with overcorrection with two-muscle surgery (p=0.008).

Conclusion: For hypertropias of 14-25pd in primary position due to unilateral fourth nerve palsies, two-muscle surgery should be approached with caution if the hyperdeviation to the palsied side is nine or less prism diopters.